The present disclosure generally relates to document processing devices and methods for operating such devices. More specifically, the present disclosure relates to methods and systems of limiting sheet skew as sheets are transported by a sheet transport roller system in a document processing device.
Document processing devices typically include one or more sets of nips used to transport media (i.e., sheets) within the device. A nip provides a force to a sheet as it passes through the nip to propel it forward through the document processing device. Depending upon the size of the sheet that is being transported, one or more nips in a set of nips might not contact the sheet as it is being transported.
FIG. 1A depicts a top view of a portion of an exemplary document processing device known in the art. As shown in FIG. 1A, the document processing device 100 includes three sets of nips 105a-c, 110a-c, and 115a-c. The first set of nips 105a-c are used to transport a sheet; the second set of nips 110a-c are used to perform sheet registration; and the third set of nips 115a-c are used to transport a sheet in a process direction. Although three nips are shown for each nip location, additional or fewer nips can be used. In some cases, additional nips are used to account for variations in sheet size during the transport or registration processes.
As shown in FIG. 1B, each nip in a set of nips, such as 115a-c, includes a drive roller, such as 125a-c, and an idler roller, such as 130a-c. A normal force is caused at each nip by loading the idler roller 130a-c. Friction with the sheet is used to produce a forward force that propels the sheet. Typically, each idler roller 130a-c is mounted independently from the other idler rollers in a set of nips. Furthermore, each idler roller 130a-c is typically loaded with a separate spring 135a-c. The springs 135a-c are used to keep the corresponding idler rollers 130a-c in contact with the corresponding drive rollers 125a-c as the sheet passes through the nip.
Using a separate spring for each idler roller can increase the cost of a document processing device, particularly when a set of nips includes 3 or more nips. Moreover, mounting each idler roller separately and using separate springs for each idler roller can result in high normal force variations between the nips. For example, if the springs have different tolerances or wear unevenly, a particular nip could apply a greater or lesser force than another nip. As such, walk and skew can result from the application of uneven normal forces among nips in a set of nips.
FIGS. 2A and 2B depict graphs of an amount of skew resulting from springs providing unequal normal forces in a conventional document processing device. As shown in FIG. 2A, a document processing device having a set of nips for which a first spring provides a 3.1% spring variation to a first idler roller and a −3.1% spring variation to a second idler roller with a nominal spring force of 4 Newtons (N) results in a skew angle of approximately 2.48×10−3 radians (2.48 milliradians) over a distance of approximately 3 meters. FIG. 2B depicts the effects of a system having a similar spring variation, but with a nominal spring force of 8 N. In such a case, the resulting skew angle is approximately 5.25 milliradians. As such, idler rollers that provide a differential normal force can significantly skew a sheet as it is being transported.